A liquid crystal panel such as a TFT-LCD panel includes an LED backlight driving circuit to drive the LED backlight in order to turn on the LED backlight and stabilize the lighting of the LED backlight. The LED backlight driving circuit employs a converter (e.g., a boost converter) to convert voltage through a predetermined switching operation.
As shown in FIG. 1, the LED backlight driving circuit includes a PWM IC 10, which is turned on/off according to a PWM_D on/off signal to generate a high-frequency pulse-type gate signal, the pulse width of which is increased or decreased according to an output current level of the LED backlight, an FET switching device 13, which switches driving voltage according to the gate signal output from the PWM IC 10, an inductor L used to reduce current ripples, a capacitor C used to constantly maintain output voltage, a diode D to restrict reverse current from the LED backlight, the LED backlight 12, which emits light according to the output voltage, a feedback block 11, which provides the level of the output current of the LED backlight 12 to the PWM IC 10, and conversion resistors R1, R2, and R3 connected to the feedback block 11 in series to convert the level of the output current into the voltage level.
In the LED backlight driving circuit having the above structure, the FET switching device 13 is open to supply voltage to the LED backlight 12 through the diode D at the active duration in which energy is supplied to the LED backlight 12 from a power supply V0. In contrast, the FET switching device 13 is conducted so that voltage is not supplied to the LED backlight 12 through the diode D at a passive duration in which energy is not transferred to the LED from the power supply V0. Therefore, the output voltage is controlled by changing the duty rate between the active duration and the passive duration at a predetermined switching frequency.
In the case of the LED backlight driving circuit, when the driving of the LED backlight driving circuit is started in the initial stage, the recognition voltage of the output current level is applied to an output current level recognition voltage terminal of the PWM IC 10 in the form of the square wave as shown in FIG. 2. Accordingly, the gate signal of a PWM IC is generated for a predetermined duty cycle when a PWM dimming signal is applied to a PWM_D terminal. However, when the gate signal of the PWM IC is generated for the predetermined duty cycle, the FET switching device is simultaneously driven by the gate signal. Accordingly, inrush current is instantly caused due to the leakage component in the converter. When the inrush current is caused at a section in which AC is cut off by the leakage component of the converter as described above, noise may be generated in an inductor and a capacitor due to the inrush current.